6.11J: Structure - Perovskite (\(CaTiO_3\))

This module will discuss aspects of the perovskite structure (CaTiO3). It will describe structural and electronic properties, give historical background, list commonly occurring substances, and describe several technological applications of this structure.

Introduction

Formally, the definition of the perovskite structure was one that has the same configuration as calcium titanate (CaTiO3).Although most perovskites have orthorhombic unit cells; or a rectangular prism structure with a rectangular base of volume axbxc, some exhibit cubic or tetrahedral geometries. Perovskites are stable at an extremely wide range of conditions and have unique chemical and physical properties, which make them an ideal material for many technological applications.

Structure

The perovskite family is a double-oxide type, mixed Ca(II)/Ti(IV) oxide with the abbreviation ABO3, where A=Ca, and B=Ti. The term "mixed" refers to the distributed nature of the oxygen ions in the unit cell, and even though the name suggests the presence of [TiO3]-2 ion, CaTiO3 does not contain it. In a unit cell, the calcium ion is located in the center of the lattice, coordinated with 12 oxygen ions, and the titanium ions are located at the corners, coordinated with six oxygen atoms. The A and B ions in the perovskite lattice are not limited to just calcium and titanium, they can be many different elements, but need to be a certain size to fit in the crystal lattice. Other requirements include that A>B, A must have an oxidation state of II, and B needs to have an oxidation state of IV. Slight variations in the relative size or oxidation state of the A or B atoms can lead to deformations, which will be discussed later. Although CaTiO3 is referred to as the "founding father" of perovskites, CaTiO3 has not been widely studied. That is probably because other perovskites such as BiTiO3, and SrTiO3 are more promising in terms of their technological applications.

The perovskite structure is versatile and robust. It can be cubic, tetrahedral, or orthorhombic at STP (standard temperature and pressure). Figure 1 depicts a typical material with orthorhombic geometry. The orthorhombic and tetrahedral geometries differ from the cubic geometry because the dimensions of the unit cell are not equal. Surprisingly, in addition to being observed at STP, the orthorhombic post-perovskite phase of MgSiO3 is found in the lower mantle, deep within the earth. This phase is stable at pressures of over 120 GPa, and at temperatures of over 2500 K. This type of perovskite is probably most abundant on earth, and might hold secrets of the geophysics of the interior of earth.

History and Typical Materials

The first perovskite type material was discovered in the Ural mountains of Russia by Gustav Rose (1839). It was later named after a Russian minerologist L.A. Perovski, who first characterized the structure.The crystal structure of a CaTiO3 type orthorhombic perovskite was first published in 1945 by Helen Dick McGaw. Research on perovskites did not catch on until the mid-40s, when there was a dramatic increase in solid-state research, especially focusing on ferroelectric materials like perovskites. Since the 1940s, perovskite materials have been a hot topic of research, and will remain a promising research frontier far into the future.

Perovskites are found naturally as kimberlites, carbonatites, and calcareous skarns (CaTiO3). More specifically, some examples of materials with perovskite structure include: CaTiO3, MgSiO3, SrFeO3, BaTiO3, SrZrO3, and the non-oxide KMgF3. These materials are commonly found in the Ural Mountains in Russia, Magnet Cove in the US, Mt.Vesuvius, Sweden, Germany and deep within the earth.

Applications

Perovskites have a wide range of applications in technology, especially in solid-state ionics. Perovskites are currently used in sensors, memory devices (RAM), amplifiers, fuel cells, superconductors, and electroptical devices. Orthorhombic perovskites such as BaZrO3 and SrZrO3are currently being developed as the electrolyte material for solid oxide fuel cells (SOFCs). Perovskites have an unusually high tolerance for oxygen ion vacancies, which makes it an ideal electrolyte or cathode material for fuel cells. Oxygen vacancies can be created in the material by doping with rare earth elements such as Y(III) or Yb(III), which replace the B atom, and create one oxygen vacancy in order to maintain charge neutrality. These randomly distributed oxygen vacancies can significantly change the conductivity of the material and make it a very efficient electrolyte.

Another current area of research involving perovskites is high-temperature superconductors (HTSCs).The cuprate-perovskite type is a specific kind of superconducting material that could have the capacity to operate under STP in the future. Cuprate-perovskites are related to CaTiO3type perovskites, but differ in that the B atoms have eight-fold coordination with oxygen atoms. Starting from the CaTiO3 prototype and going to YBa2Cu3O7, Ba and Y substitute for Ca, while Cu substitutes for Ti. Some cuprate-perovskite ceramic HTSCs currently have an operating temperature of 90 K, which is significantly higher than most other superconducting materials. However, much work needs to be done before a superconductor can operate at 273 K. Figure 2 shows the idealized orthorhombic unit cell of Sr2RuO4. In reality, the octahedra composed of Ru and O would be slightly tilted with respect to each other.

Some other unique properties that sets perovskites apart and makes them ideal for technological applications include: it is the only crystal structure that is ferroelectric (spontaneous alignment of the electric dipoles caused by interactions between them) not because of an external magnetic field but due to its crystal structure, its ferro-, pyro-, and piezo-electric properties, and structural properties such as durability and chemical flexibility.

Q: Given that perovskites in an orthorhombic configuration have lattice constants of roughly a=5.4338, b=5.4886, and c=7.6841 angstroms(A), and using the figures above, determine the density. Each calcium ion is located at the center of the octahedra and has 12-fold coordination with the oxygen, and the titanium ions are located at the corners and has six-fold coordination with oxygen.

A: The orthorhombic configuration, which is the most common configuration in perovskite materials, has lattice constants of a=5.4338, b=5.4886, and c=7.6841 angstroms. One orthorhombic unit cell contains four A's, which are the calciums for example, four B's and 12 O's. For the case of CaTiO3, using the lattice constants and the number of atoms per cell listed previously, the density of orthorhombic CaTiO3 is 3.944 g/cm3. A sample calculation is provided below.